1. Field of the Invention
The present invention relates to a passive optical network system with a power saving function, an optical line terminal, and an optical network unit.
2. Description of the Related Art
There are increasing demands for a high speed transmission and a broader band communication in network communication. To meet such demands, introduction of optical networks are planed. The Optical network is a network providing a point-to-point communication between an optical line terminal (hereinafter referred to as OLT) and an optical network unit (hereinafter referred to as ONU) through optical fibers. The passive optical network (hereinafter referred to as PON) forms a network providing a star type of point-to-multipoint communication between one OLT and a plurality of the ONUs through an optical fiber and a beam splitter for dividing the optical fiber.
As a typical standard of the PON, there are EPON (Ethernet (registered trade mark) PON) standardized by IEEE802.3 and GPON (Gigabit Capable PON) standardized by ITU-T recommendation G.984.
In the PON, upstream frames transmitted to the OLT from ONU and the downstream frames transmitted from the OLT to the ONU are wavelength-division-multiplexed (hereinafter referred to as WDM). In the downstream frames, the same data is transmitted to all ONUs connected to the OLT with the optical fibers. The ONU having received the data neglects data in frames other than a frame directed to the ONU itself by referring destination information included in the preamble of the data and forwards the data in only the frame directed to the optical network itself to a user side. On the other hand, the upstream frames are multiplexed by Time Division Multiplexing (hereinafter referred to as TDM) in which each ONU transmits the data in a time section individually specified by the OLT.
A communication speed of the PON ranges from a system using a low data rate (speed) such as 64 kbit/sec, to the BPON (Broad band PON) for transmitting and receiving ATM (Asynchronous Transfer Mode) cells having a fixed data length at a maximum 600 Mbit/sec. In addition, the EPON for transmitting and receiving packets of Ethernet having a variable data length at approximately 1 Gbit/sec at maximum, and the GPON using a signal having a higher data rate of about 2.4 Gbit/sec, have been introduced. Hereinafter a higher data rate PON capable of using a signal having data rate of 10 Gbit/sec to 40 Gbit/sec is required to be realized. With increase in the communication rate (speed) in the PON, a power consumption of a relay unit on the transmission path tends to be larger. Because the ONU is installed in a subscriber's house, a lot of ONUs are installed on the network. On the other hand, the ONU requires a shorter time interval of a usable bandwidth than the OLT or an upper rank switch groups. Accordingly, the ONU is left unchanged though the ONU uses a useless power also during non-communication.
As a prior art relating this, JP 2008-113193 discloses a method of reducing the power consumption by setting the functional block inside the ONU to a low power consumption mode when a terminal equipment (TE) is not connected to LAN (Local Area Network) cable. JP2009-260970 discloses a method of transmitting a sleep request from the ONU to the OLT and setting the ONU in a sleep status by a procedure of issuing permission for the request by the OLT.
When a power failure or a power saving are forced by a widespread disaster or a failure, etc. in a power supplying function, the electric power supplying capacity for operating the optical access transmission equipment becomes lack, so that operation of the equipment cannot be maintained. Accordingly, there occurs a state where even minimum communication function, which is really necessary, cannot be provided. Because the communication network is a lifeline for supporting smooth civic life, it is important to meet the communication request in emergency by efficiently using a limited power though it cannot be avoided that a part of functions may decrease in the above-described abnormal state. Particularly, a ratio of electric power consumed by the optical access equipments in the whole of the communication network is large, so that it is important to operate the optical access equipment with a smaller power consumption.
As described above, as a higher speed and a larger capacity communication having been developed, there is an increasing trend in the power consumption of the relay terminal on the transmission line becomes large, In contrast, it can be supposed that decreasing the transmission rate can reduce the power consumption. However, because the existing PON are designed to operate a preset clock frequency, both JP 2008-113193 and JP 2009-260970 do not disclose that the transmission rate is decreased in accordance with the power supplying state. Particularly, it is necessary to avoid a status requiring time for re-transmitting a user signal because a user signal may be dropped out while the transmission rate is switched. In addition, in consideration of differences, for each ONU, in the power supplying state, the capacity of a battery installed in pair with each of the ONUs, a service content supported by the ONU, the number of end users, and the transmission clock, it is desired to change the transmission clock for each ONU.